This invention relates generally to gas turbine engines and, more particularly, to an improved method and apparatus for forming film cooling holes in the turbine airfoils of a gas turbine engine.
In order to permit turbine nozzle guide vanes and rotor blades to operate at the extremely high gas stream temperatures which are desirable for increased efficiency, it is necessary to cool the hollow airfoil with the use of high pressure cooling air from the compressor. One of the most effective modes, and least demanding of cooling air volume, is that of film cooling wherein a layer of cooling air is maintained between the high temperature gases and the external surfaces of the turbine blades and vanes. The layer of cooling air is formed by directing airflow from within the airfoil through a series of small holes which are formed in the airfoil in the desired pattern. The resulting film of air which flows over the airfoil surface tends to reduce peak metal temperatures and temperature gradients to thereby make the use of higher turbine inlet temperature ranges possible, which in turn increases the engine efficiency.
It has been found that film cooling effectiveness with reduced amounts of cooling air use can be obtained by the use of "diffusion" film holes, wherein the discharge side of the gill hole is tapered to an increased opening size by an angle of up to 7.degree. from the axis of the hole. However, even though this phenomenon has been recognized for its performance advantages, the forming of such diffusion gill holes has been relatively expensive and poor in quality. Various methods devised for this purpose include the practice of introducing a dwell time in an electrostream drilling operation, a multiple pass process using the electrochemical machining (ECM) or electrodischarge machining (EDM) process and a laser drilling process with multliple discharges of energy. Electrostream with dwell is achieved by having the instrument pause at a particular point on its path of movement toward the workpiece to be drilled so as to thereby erode a diffused area at the exit end of the gill hole. Such a practice is effective but prohibitively expensive. In the case of electro-discharge machining, which is a primary process for putting film holes in nozzle vanes and bands, there has been no cost effective way of providing for these diffusion film holes. A multiple pass process forms the straight portion of the hole with one process step and forms the diffuser portion with a separate process step. This adds costs and presents quality problems because the two portions of the film hole tend to misalign and spoil the desired smooth diffusion of the cooling fluid.
Another method, wherein three-dimensionally shaped electrodes are used for each individual hole, is considered prohibitively expensive.
It is, therefore, an object of the present invention to provide an improved method and apparatus for foming diffused film holes in an airfoil.
Another object of the present invention is the provision for an economical and effective method of forming diffusion film holes in an airfoil.
Yet another object of the present invention is the provision for forming diffusion film holes in a workpiece by way of an improved electro-discharge machining method and apparatus.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjuction with the appended drawings.